1. Field of the Invention
The present invention relates to a resin composition for extrusion molding. More particularly, it relates to a resin composition for extrusion molding which is used for extrusion molding of a packaging film or sheet.
2. Description of the Related Art
Polyolefin resins such as low density polyethylenes, ethylene-vinyl acetate copolymers, ionomers, etc., can be heat sealed at relatively low temperatures and have high heat-sealing strength. Therefore, they are often used as a heat sealing layer in a packaging film in the form of a mono layer or a multi-layer with other thin film material. The laminated films are usually prepared by a process such as co-extrusion, co-extrusion coating, dry laminating, etc.
Recently, high speed packaging is increasingly required in an automatic packaging system. The high speed packaging means that heat sealing is completed in a short time, and a period of time from the heat sealing to the application of a weight of a content in a bag to the heat-sealed portion is shortened. For this purpose, there is required a film in which heat-sealing properties can be developed at a low temperature and no peeling at the sealed portion occurs even if the weight of the content is applied to the sealed portion when it remains hot immediately after heat sealing, that is, a film having excellent hot tacking properties.
In laminated films, the heat sealing properties and the hot tacking properties depend to a large extent on the resin used for formation of a heat sealing layer while selection of a substrate film is also important.
Among the polyolefin resins, the ethylene-vinyl acetate copolymer is widely used as the heat sealing layer since it exhibits excellent heat sealability at low temperature in comparison with the low density polyethylene or the ionomers. However, it is poor in hot tacking properties and not satisfactory in heat-sealing strength.
Japanese Patent Kokoku No. 41529/1987 describes that, when a resin composition for extrusion molding comprising (a) a low density polyethylene, (b) an ethylene-.alpha.-olefin copolymer and (c) a low crystalline to non-crystalline ethylene-.alpha.-olefin copolymer is used as a heat sealing layer, it satisfies the properties required for the heat sealing layer in packaging materials, for example, heat sealability at low temperature, heat-sealing strength, hot tacking, strength in bag breaking. However, concerning heat-sealing strength, hot tacking and strength in bag breaking, it does not sufficiently satisfy the recent requirement for the heat sealing layer.